Signal transmitting cable

ABSTRACT

An optical fibre cable ( 1 ) for installation in a duct ( 51 ) by means of fluid flow is described. The cable ( 1 ) has a signal transmitting portion comprising a plurality of elongate, flexible optical fibres, surrounded by a braided covering ( 10 ) formed from textile yarns ( 9 ). The apparatus ( 50 ) for installing the cable includes an air compressor ( 52 ) for supplying a blowing head ( 53 ) with a conditioned air supply substantially free of static electricity or moisture. The blowing head ( 53 ) includes drive wheels (not shown), the speed of rotation of which (and hence the speed of installation of the fibre cable ( 1 ), ( 101 ) is adjusted by means of s control unit ( 54 ) powered by power supply ( 55 ). The fibre cable ( 1 ), ( 101 ) is fed from a fibre pan ( 56 ) over guide ( 57 ) to the blowing head ( 53 ), which is fed with an air supply from compressor ( 52 ) which is regulated by dispenser ( 58 ).

[0001] The present invention relates to signal transmitting cables forinstallation in ducts, and relates particularly, but not exclusively, tooptical fibre light signal transmitting cables for installation intounderground ducts or into buildings.

[0002] It is common to transmit telecommunication signals by means ofoptical fibre cables installed in underground ducts. In order tominimise the number of persons needed to install such cables, andtherefore minimise the installation cost, it is known to install cablesconsisting of bundles of flexible optical fibres into ducts by means ofcompressed air. An end of the cable is inserted into one end of a duct,and the cable is then blown into the duct by means of compressed air,fluid drag between the moving air and the surface of the cable causingthe cable to move axially along the duct. It is known to provide suchcables consisting of optical fibres encapsulated in a thin plasticsheath, which is then encapsulated in a thicker foam plastic layer. Thefoam plastic layer makes the cable relatively light in weight, butprovides it with a large surface area, giving good fluid drag as airpasses over it in the installation process.

[0003] However, cables of this type, known to persons skilled in the artas MK I cables, suffer from the drawback that because the cable isrelatively large in diameter, it is necessary to use a large diameterduct for installation of the cable in order to achieve commerciallyacceptable installation distances. In the case of telecommunicationscables being installed in duct networks, especially in city centrelocations where space in underground ducts is scarce because of the veryhigh number of customers and the high cost of construction, it isundesirable to use large ducts. In addition, MK I cables usually usepolyethylene foam outer layers, which have relatively poor frictioncharacteristics when in contact with the internal surface of theinstallation tube, which in turn restricts the blowing distancesachievable with this product.

[0004] An attempt to solve this problem, known to persons skilled in theart as an MK II cable, is disclosed in International patent applicationnumber Wo 93/01512, UK Patent application number GB 2282897, andEuropean patent application numbers EP 0422764, EP 0752604 and EP0345968. In this type of cable, the optical fibres are coated in a twostage process, and the outer layer contains microscopic glass spheresdesigned to provide a rough surface to enhance fluid drag, whileproviding a low friction contact surface between the optical fibre cableand the tube. The use of microscopic glass spheres means that the outerlayer can be significantly thinner than the foam outer layer of the MK Icable, and the improved friction performance means that this type ofprior art cable can be installed over commercially satisfactorydistances in smaller diameter ducts.

[0005] However, there has persisted a commercial requirement for opticalfibre cables which are capable of installation by blowing over longerdistances than are possible with the MK II cable, since if the number ofoptical fibre splices in a cable network can be reduced, theinstallation time and cost can be reduced and the process can be madesuitable for a wider range of applications.

[0006] Preferred embodiments of the present invention seek to provide asignal transmitting cable which can be blown into a duct over longerdistances than in the prior art.

[0007] According to an aspect of the present invention, there isprovided a signal transmitting cable for installation in a duct by meansof fluid flow, the cable comprising:

[0008] a signal transmitting portion comprising at least one elongate,flexible, signal transmitting member; and

[0009] a covering surrounding said signal transmitting portion andhaving on the outer surface thereof at least two sets of variations indiameter, including a first set of protrusions and/or depressions and asecond set of protrusions having respective first and second averageamplitudes in a radial direction of the signal transmitting portion andrespective first and second average spacing in a respective furtherdirection along the surface of said covering;

[0010] wherein said second average amplitude is greater than said firstaverage amplitude, and said second average spacing is greater than saidfirst average spacing.

[0011] By providing a first set of protrusions or depressions and asecond set of protrusions, where the average amplitude and spacing ofthe second set is greater than those of the first set, this provides thehighly surprising and advantageous effect of significantly increasingthe blowing performance of a signal transmitting cable compared with theprior art cables. It is believed that this effect is achieved by meansof the second set of protrusions minimising the area of contact betweenthe signal transmitting cable and the internal surface of the tube (thusminimising friction between the cable and the tube), and the first saidof protrusions or depressions at the same time increasing fluid dragbetween the cable and the compressed fluid blowing the cable into thetube.

[0012] At least one said further direction may be substantially parallelto the axis of said signal transmitting portion.

[0013] At least one said further direction may extend helically aroundsaid signal transmitting portion.

[0014] The first average spacing is preferably arranged such that theair drag imparted to a section of the cable in use during blowing into atube exceeds

A _(a)(P ₁ −P ₂)S _(c)/(S _(c) +S _(d)),

[0015] where

[0016] A_(a) is the average cross-sectional area of fluid flowingbetween the section of the cable and the tube;

[0017] (P₁−P₂) is the fluid pressure difference between the ends of thesection of cable;

[0018] S_(c) is the average cross-sectional external perimeter length ofthe section of cable; and

[0019] S_(d) is the average cross-sectional internal perimeter length ofthe tube surrounding the section of cable.

[0020] In a preferred embodiment, said first average spacing is lessthan substantially 30 mm.

[0021] In a preferred embodiment, the covering comprises at least onecoating of material, and said first set of protrusions or depressions isprovided on the outermost said coating.

[0022] At least one said coating may be of plastics material.

[0023] A said first set of protrusions may be printed on the outermostsaid coating.

[0024] In a preferred embodiment, the covering comprises at least oneintermittent coating of material, and said second set of protrusions isprovided on the outermost said intermittent coating.

[0025] At least one said intermittent coating may comprise plasticsmaterial.

[0026] The second set of protrusions may comprise variations in diameterof the outermost said intermittent coating.

[0027] The variations in diameter may be formed by varying the pressureof said plastics material during formation of the outermost saidintermittent coating.

[0028] The outermost said intermittent coating may be formed byspraying.

[0029] In a preferred embodiment, the covering is at least partiallyformed by extrusion.

[0030] The covering may be at least partially formed from crossedtextile fibres.

[0031] The covering may include fibres of different lateral thicknesses.

[0032] The covering may include a mixture of fibres of substantiallyflattened and substantially circular cross section.

[0033] The covering may include fibres of different diameters.

[0034] The covering may be braided.

[0035] The covering may be woven.

[0036] The covering may be knitted.

[0037] At least one said fibre may be formed from a plurality offilaments.

[0038] In a preferred embodiment, at least one said signal transmittingmember is an optical signal transmitting member.

[0039] The or each said optical signal transmitting member may be anoptical fibre.

[0040] At least one said signal transmitting member may be a conductorfor transmitting an electrical signal.

[0041] The cable preferably further comprises at least one sheathsurrounding at least one said signal transmitting member.

[0042] The cable may comprise a plurality of said signal transmittingmembers, and an adhesive layer arranged between at least one pair ofadjacent said signal transmitting members.

[0043] The second set of protrusions may be provided with a frictionreducing coating.

[0044] According to another aspect of the invention, there is provided amethod of forming a signal transmitting cable as defined above, themethod comprising forming said covering around said signal transmittingportion.

[0045] The step of forming said covering may include forming at leastone coating of material around said signal transmitting portion, andproviding said first set of protrusions and/or depressions on theoutermost said coating.

[0046] A first set of said protrusions may be formed on said outermostcoating by means of printing.

[0047] The step of providing said second set of protrusions on theoutermost said coating may comprise varying the pressure of saidmaterial during formation of said coating.

[0048] The pressure of said material may be varied by means of a gearpump.

[0049] The pressure of said material may be varied by means of at leastone solenoid valve.

[0050] The step of forming said outermost coating may comprise sprayingsaid outermost coating onto the cable.

[0051] The step of forming said covering may comprise at least partiallyextruding said covering.

[0052] The step of forming said covering around said signal transmittingportion may comprise forming said covering of crossed textile fibres.

[0053] The step of forming said covering may comprise braiding saidtextile fibres.

[0054] The step of forming said covering may comprise forming saidcovering from textile fibres of different lateral thicknesses.

[0055] The step of forming said covering may comprise forming saidcovering from textile fibres of different diameters.

[0056] The step of forming said covering may comprise forming saidcovering from a mixture of fibres of substantially flattened andsubstantially circular cross section.

[0057] The method preferably further comprises the step of providingsaid second set of protrusions with a friction reducing coating.

[0058] The first set of protrusions and/or depressions and/or saidsecond set of protrusions may be arranged helically around saidcovering.

[0059] Preferred embodiments of the invention will now be described, byway of example only and not in any limitative sense, with reference tothe accompanying drawings, in which:—

[0060]FIG. 1 is a side, perspective view of a fibre optic cable of afirst embodiment of the present invention;

[0061]FIG. 2 is a schematic view of an apparatus for use in blowing thecable of FIG. 1 into a duct;

[0062]FIG. 3 is a schematic illustration of a track used in ComparativeExample 1;

[0063]FIG. 4 is a graph of comparative blowing performance inComparative Example 1;

[0064]FIG. 5 is a schematic view of an apparatus for forming an opticalfibre cable of a second embodiment of the present invention;

[0065]FIG. 6 is a schematic cross-sectional view of a duct containing anoptical fibre cable formed by the apparatus of FIG. 5;

[0066]FIG. 7 is a cross sectional view of an optical fibre cable of athird embodiment of the present invention;

[0067]FIG. 8 is a cross sectional view of an optical fibre cable of afourth embodiment of the present invention; and

[0068]FIG. 9 is a schematic representation of an annulus of fluidlocated between the optical fibre cable of the invention and a duct, foruse in calculating the fluid drag on the cable.

[0069] Referring to FIG. 1, an optical fibre cable 1 of a firstembodiment of the invention is made by taking a cable containing twelveoptical fibres 3, encapsulated in a plastic sheath 2, for example aprior art MK II cable manufactured in accordance with the processdescribed in the prior art documents mentioned above. The fibre bundleis pulled from a fibre pan through an orifice 4 of a braiding head (notshown) having two counter-rotating sinusoidal tracks;, as will befamiliar to persons skilled in the art.

[0070] The braiding head is provided with counter rotating sinusoidaltracks, each of which carries yarn carriers carrying textile yarns, theyarns being arranged in an intertwined configuration, and is providedwith an eyelet arranged centrally above the yarn carriers. The yarns arepassed though the eyelet to converge and are then passed through gearedcapstans to form a braided covering 10 by means of a braid known as amaypole braid. In the arrangement shown in FIG. 1, the braided covering10 formed contains a mixture of light multifilaments 9 and heavymonofilaments 11. Ten fibres of such multifilament 9 are used, as wellas two 180 Denier continuous monofilament nylon yarns 11. The 180 Denieryarn is approximately three times the lateral thickness of the 140D/tex,multifilament. The braided covering 10 formed by this process hasa number of small amplitude protrusions at the crossing points of themultifilaments 9, and a number of larger amplitude protrusions at thecrossing points of the monofilaments 11. It can be seen that the axialspacing of the large amplitude protrusions is greater than that of thesmall amplitude protrusions.

COMPARATIVE EXAMPLE 1.

[0071]FIG. 2 shows an apparatus 50 for installing the optical fibrecable 1 of FIG. 1 into a duct 51. The apparatus 50 includes an aircompressor 52 for supplying a blowing head 53 with a conditioned airsupply substantially free of static electricity or moisture. The blowinghead 53 includes drive wheels (not shown), the speed of rotation ofwhich (and hence the speed of installation of the fibre cable 1) isadjusted by means of a control unit 54 powered by power supply 55. Theoptical fibre cable 1 is fed from a fibre pan 56 over guide 57 to theblowing head 53, which is fed with an air supply from compressor 52which is regulated by dispenser 58.

[0072] The blowing head 53 is also provided with a seal 59 to minimiseair leakage through the back of the blowing head during installation,and a blowing bead 60 is attached to the forward end of the fibre cable1 before blowing, to guide the cable through any connectors and sharpbends of the duct 51, which is clamped to the blowing head 53. A flowmeter 61 is provided at the end of the duct 51 remote from the blowinghead 53 to measure the rate of air flow.

[0073] Referring now to FIG. 3, 1 km lengths L2 of the fibre cable 1 ofFIG. 1 were blown through a duct 51 of outside diameter 5 mm and insidediameter 3.5 mm along two laps of the route shown. A braided opticalfibre cable L1 having a braided coating made from yarns of equalthickness was also blown through the duct. Table 1 provides details ofbends provided on the track. An uncoated MKII fibre optic cable L3 wasalso blown along the same route.

[0074]FIG. 4 shows the results of blowing performance, on which curve L3shows that the installation speed of the conventional MKII fibre opticcable started to reduce after an installation distance of approximately400 m, and then rapidly declined. Curve,L1, on the other hand, showsthat the fibre optic cable having yarns of a single thickness was stilltravelling with an installation speed of 15 m per minute after a blowingdistance of 1000 m. However, and curve L2 shows that the cable 1 of FIG.1 achieved even better blowing performance. It can therefore be seenthat providing a covering of braided textile fibres of mixed thicknessessignificantly improves the blowing performance of the cable.

[0075] Table 1 shows the coefficient of friction and air drag for thethree fibre optic cables of FIG. 4. TABLE 1 Coefficient Air drag atCable of friction 2 Bar (gf) L1 0.12 72 L2 0.09 84 L3 0.1 36

[0076] Referring now to FIG. 5, an apparatus 101 for forming an opticalfibre cable 102 of a second embodiment of the invention includes banks103 of tension controlled fibre payoff reels 104 delivering a total oftwelve primary coated optical fibres 105, i.e. optical fibres comprisinga light transmitting portion surrounded by a protective sheath.

[0077] The optical fibres 105 pass over guide wheels 106 and dancers forcontrolling the tension in the optical fibres 105, around a guide wheel107 and are then coated with liquid thermoplastic material (for exampleepoxy acrylate material) in a coating unit 108 to form a continuouscoating on the optical fibres 105. The coated fibre bundle 111 is thenfed into a printing unit 109 having a pair of opposed rollers 110 havingslots 112 which apply ink to the optical fibre bundle 111. This producesa series of bands or protrusions 113 which protrude radially outwardsfrom the optical fibre bundle 111. The ink and coating are then cured bypassage past an ultraviolet lamp 114. Alternatively, the coating may becured prior to application of the ink in the printing unit 109.

[0078] After curing by the ultraviolet lamp 114, the bundle 111 passesthrough a further coating section 115 where it is coated intermittentlywith liquid thermoplastic resin material such as epoxy acrylatematerial. A gear pump or bank of solenoid valves with mutually offsetcycles (not shown) is used to create pulsed pressure variations in theliquid resin material, which causes beads 116, of greater amplitude thanthe printed protrusions 113 and spaced further apart than the printedprotrusions 113, to be formed in the intermittent coating formed on thebundle 111. Alternatively, the intermittent coating forming the beads116 may be sprayed onto the bundle 111 in the further coating section115, and may include a friction reducing component. The coating appliedto the bundle 111 is then cured by a second ultraviolet lamp 117, andthen passed around a guide roller 118.

[0079] It can be seen from FIG. 6 that the coated optical fibre bundle111 has a series of closely spaced protrusions 113 of small amplitudeand beads 116 of larger amplitude spaced further apart than the closelyspaced protrusions 113. It is believed that in blowing the fibre bundle111 through a duct 119 by means of compressed air 120, the protrusions113 increase the fluid drag acting on the fibre bundle 111, and thebeads 116 reduce the points of frictional contact with the wall of theduct 119. It will be appreciated by persons skilled in the art that FIG.6 is provided for illustrative purposes only, and that the dimensions ofthe variations in diameter of the bundle 111 are greatly exaggerated.

[0080]FIG. 7 shows a cross sectional view of an optical fibre cable 201of a third embodiment of the invention. The cable 201 includes a signaltransmitting portion comprising four coated optical fibres 202, and acoating 203 of plastics material formed around the signal transmittingportion. The coating 203 has a set of small protrusions 204 forincreasing the fluid drag on the cable 201 when it is blown into a duct(not shown), and a set of large protrusions 205 for minimisingfrictional contact with the internal wall of the duct. The coating 203is formed by means of extrusion around the signal transmitting portionby extrusion through a die (not shown) having an internal profilecomplementary to the external profile of the coating 203. In this way,the small 204 and large 205 protrusions are formed in a single step.

[0081]FIG. 8, in which parts common to the embodiment of FIG. 7 aredenoted by like reference numerals but increased by 100, shows a crosssectional view of a cable 301 of substantially identical construction tothe cable 201 of FIG. 7, but has a coating 303 formed in two steps. Inthe first step, the coating 303 is formed radially outwards as far asdotted line 306, and that part of the coating radially outwards of theline 306, including smaller 304 and larger 305 protrusions, is formed ina second extrusion step. In this way, the coating 303 can be formed fromtwo separate materials, for example a material having suitableproperties of flexibility for radially inwards of the line 306, and amaterial having suitable properties for formation of the protrusions304, 305.

[0082] Referring again to FIG. 6, the dimensions and spacing of theprojections 113 necessary to achieve improved blowing performance aredetermined according to the following relationship, where δ_(L) is theheight of a laminar layer of compressed air above the surface of anoptical fibre bundle 111 not provided with protrusions 113, and e is theaverage projection height of a protrusion 113 above the surface of anoptical fibre bundle 111 not having protrusions 113:

[0083] for entirely smooth flow δ_(L)<6e, so for roughness to have anaffect δ_(L/6)<e;

[0084] for rough flow δ_(L)/0.3<e.

[0085] δ_(L), the height of the laminar layer above the optical fibrebundle 111 is further given by the following relationship$\begin{matrix}{\delta_{L} = \frac{32.8D}{{Re}\sqrt{\quad}f}} & {{Equation}\quad 1}\end{matrix}$

[0086] where R_(e), Reynolds number, is given by

[0087] R_(e)=DV/u, and D=hydraulic diameter

[0088] f=fluid friction factor, defined as${f = \frac{h_{L}}{\left( {L/D} \right)V^{2/}2g}},$

[0089] where

[0090] g=acceleration due to gravity

[0091] h_(L)=fluid head loss L=length of duct and fibre combinationu=kinematic viscosity of fluid V=general fluid velocity. In practicalsituations, case 1 in Table 2 represents the highest value of thedenominator of equation 1 typically encountered, and case 2 representsthe lowest value typically encountered. TABLE 2 Denominator R_(e) f(R_(e{square root}f)) Case 1 40000 0.2 17888.54382 Case 2 2000 0.032357.7708764

[0092] A method of estimating the fluid drag acting on the cable 102 ofFIG. 6 is described with reference to FIG. 9, which shows a thin annulusof fluid located between the cable 102 and the duct 119 into which thecable 102 is being blown.

[0093] In the case of the cable 102 being blown into the duct 119 atuniform speed, the force acting on the annulus due to the pressure dropacross the annulus will be equal to the forces exerted by the annulusdue to fluid friction. In addition, the fluid drag acting on the cable102 will be proportional to the area of that part of the cablesurrounded by the annulus. In other words:

[0094] δF=A_(a). (area of section of cable)/(area of section ofcable+duct),

[0095] i.e.δF=A_(a).δp×δA_(c)/(δA_(c+)δA_(d))=A_(a).δp×S_(c)/(S_(a)+S_(c)).

[0096] It can therefor be seen that the total force acting on a sectionof cable is given by:

F=∫ ^(p1) _(p2) dF=A _(a) S _(c)(p ₁ −p ₂)/(S _(c) +S _(d)),

[0097] where

[0098] A_(a) is the cross sectional area of the annulus;

[0099] A_(c) is the surface area of the cable section;

[0100] A_(d) is the internal area of the duct surrounding the cablesection;

[0101] F is the fluid drag acting on the cable section;

[0102] p is the fluid pressure in the duct;

[0103] S_(c) is the cross sectional perimeter length of the cablesection; and

[0104] S_(d) is the cross sectional perimeter length of the ductinternal wall.

[0105] It will be appreciated by persons skilled in the art that theabove embodiments have been described by way of example only, and not inan limitative sense, and that various alterations and modifications arepossible without departure from the scope of the invention as defined bythe appended claims. For example, the protrusions have been described interms of their average separation in an axial direction of the cable,but it will be understood that the protrusions could be arrangedhelically around the cable, for example in a so-called S-Z arrangementin alternating senses to reduce any turning moment applied to the cableon blowing into the duct.

1. A signal transmitting cable for installation in a duct by means offluid flow, the cable comprising: a signal transmitting portioncomprising at least one elongate, flexible, signal transmitting member;and a covering surrounding said signal transmitting portion and havingon the outer surface thereof at least two sets of variations indiameter, including a first set of protrusions and/or depressions and asecond set of protrusions having respective first and second averageamplitudes in a radial direction of the signal transmitting portion andrespective first and second average spacing in a respective furtherdirection along the surface of said covering; wherein said secondaverage amplitude is greater than said first average amplitude, and saidsecond average spacing is greater than said first average spacing.
 2. Acable according to claim 1, wherein at least one said further directionis substantially parallel to the axis of said signal transmittingportion.
 3. A cable according to claim 1 or 2, wherein at least one saidfurther direction extends helically around said signal transmittingportion.
 4. A cable according to any one of the preceding claims,wherein said first average spacing is arranged such that the air dragimparted to a section of the cable in use during blowing into a tubeexceeds A _(a)(p ₁ −p ₂)S _(c)/(S _(c) +S _(d)), where A_(a) is theaverage cross-sectional area of fluid flowing between the section of thecable and the tube;. (p₁−p₂) is the fluid pressure difference betweenthe ends of the section of cable; S_(c) is the average cross-sectionalexternal perimeter length of the section of cable; and S_(d) is theaverage cross-sectional internal perimeter length of the tubesurrounding the section of cable.
 5. A cable according to any one of thepreceding claims, wherein said first average spacing is less thansubstantially 30 mm.
 6. A cable according to any one of the precedingclaims, wherein said covering comprises at least one coating ofmaterial, and said first set of protrusions and/or depressions isprovided on the outermost said coating.
 7. A cable according to claim 6,wherein at least one said coating is of plastics material.
 8. A cableaccording to claim 6 or 7, wherein a said first set of protrusions isprinted on the outermost said coating.
 9. A cable according to any oneof the preceding claims, wherein said covering comprises at least oneintermittent coating of material, and said second set of protrusions isprovided on the outermost said intermittent coating.
 10. A cableaccording to claim 9, wherein at least one said intermittent coatingcomprises plastics material.
 11. A cable according to claim 9 or 10,wherein said second set of protrusions comprises variations in diameterof the outermost said intermittent coating.
 12. A cable according toclaim 11, wherein said variations in diameter are formed by varying thepressure of said plastics material during formation of the outermostsaid intermittent coating.
 13. A cable according to claim 11, whereinthe outermost said intermittent coating is formed by spraying.
 14. Acable according to any one of claims 1 to 6, wherein said covering is atleast partially formed by extrusion.
 15. A cable according to any one ofclaims 1 to 6, wherein said covering is at least partially formed fromcrossed textile fibres.
 16. A cable according to claim 15, wherein thecovering includes fibres of different lateral thicknesses.
 17. A cableaccording to claim 16, wherein the covering includes a mixture of fibresof substantially flattened and substantially circular cross section. 18.A cable according to any one of claims 15 to 17, wherein the coveringincludes fibres of different diameters.
 19. A cable according to any oneof claims 15 to 18, wherein the covering is braided.
 20. A cableaccording to any one of claims 15 to 18, wherein the covering is woven.21. A cable according to any one of claims 15 to 18, wherein thecovering is knitted.
 22. A cable according to any one of claims 15 to21, wherein at least one said fibre is formed from a plurality offilaments.
 23. A cable according to any one of the preceding claims,wherein at least one said signal transmitting member is an opticalsignal transmitting member.
 24. A cable according to claim 23, whereinthe or each said optical transmitting member is an optical fibre.
 25. Acable according to any one of the preceding claims, wherein at least onesaid signal transmitting member is a conductor for transmitting anelectrical signal.
 26. A cable according to any one of the precedingclaims, further comprising at least one sheath surrounding at least onesaid signal transmitting member.
 27. A cable according to any one of thepreceding claims, comprising a plurality of said signal transmittingmembers, and an adhesive layer arranged between at least one pair ofadjacent said signal transmitting members.
 28. A cable according to anyone of the preceding claims, wherein said second set of protrusions isprovided with a friction reducing coating.
 29. A signal transmittingcable for installation in a duct by means of fluid flow, the cablesubstantially as hereinbefore described with reference to theaccompanying drawings
 30. A method of forming a signal transmittingcable according to any one of the preceding claims, the methodcomprising forming said covering around said signal transmittingportion.
 31. A method according to claim 30, wherein the step of formingsaid covering includes forming at least one coating of material aroundsaid signal transmitting portion, and providing said first set ofprotrusions and/or depressions on the outermost said coating.
 32. Amethod according to claim 31, wherein a said first set of protrusions isformed on said outermost coating by means of printing.
 33. A methodaccording to claim 31 or 32, wherein the step of providing said secondset of protrusions on the outermost said coating comprises varying thepressure of said material during formation of said coating.
 34. A methodaccording to claim 33, wherein the pressure of said material is variedby means of a gear pump.
 35. A method according to claim 33, wherein thepressure of said material is varied by means of at least one solenoidvalve.
 36. A method according to claim 31, wherein the step of formingsaid outermost coating comprises spraying said outermost coating ontothe cable.
 37. A method according to claim 31, wherein the step offorming said covering comprises at least partially extruding saidcovering.
 38. A method according to claim 30, wherein the step offorming said covering around said signal transmitting portion comprisesforming said covering of crossed textile fibres.
 39. A method accordingto claim 38, wherein said step of forming said covering comprisesbraiding said textile fibres.
 40. A method according to claim 38 or 39,wherein the step of forming said covering comprises forming saidcovering from textile fibres of different lateral thicknesses.
 41. Amethod according to any one of claims 38 to 40, wherein the step offorming said covering comprises forming said covering from textilefibres of different diameters.
 42. A method according to claim 40 or 41,wherein the step of forming said covering comprises forming saidcovering from a mixture of fibres of substantially flattened andsubstantially circular cross section.
 43. A method according to any oneof claims 30 to 42, further comprising the step of providing said secondset of protrusions with a friction reducing coating.
 44. A methodaccording to any one of claims 30 to 44, wherein said first set ofprotrusions and/or depressions and/or said second set of protrusions arearranged helically around said covering.